When a superconducting coil quenches upon electrification and changes from a superconducting state to a normal conductive state, the superconducting coil is damaged if the electrification state is continued. Therefore, it is necessary to stop the electrification by detecting generation of the quenching at an early stage (refer to Patent Documents 1 and 2).
In the superconducting coil electrified by a current source, the simplest quench detection method is a method for monitoring an absolute value of a voltage between both ends of the superconducting coil. This method detects a change of an output voltage of the current source along with generation of resistance in the superconducting coil. Although this method is simple, it is necessary to set a detecting voltage so as not to erroneously detect an induction voltage at the time of magnetization and demagnetization of the superconducting coil. Therefore, in many cases, the method is applied to a small superconducting coil in which a magnetization voltage is around several volts, and at the same time, the detecting voltage can be set to be several volts or more. In general, both ends of the superconducting coil are shorted by a semiconductor element and resistance for protection. Therefore, the maximum voltage input to a quench detection device is determined by the voltage between both ends of the resistance and the semiconductor element.
A method widely used in a case where the detecting voltage at the time of the quenching desires to be set small in comparison with the induction voltage at the time of the magnetization and the demagnetization of the superconducting coil is a method for monitoring a voltage difference between both sections obtained by dividing the superconducting coil. In a case where the superconducting coil is configured with a plurality of element coils, the superconducting coil may be divided at the middle point of the element coil group. In this method, a voltage difference between two sections obtained by equally dividing an inductance is monitored. Thus, generation of a minute abnormal voltage can be detected by canceling the induction voltage. In practical use, a difference in the inductance between each divided section needs to be finely adjusted on a quench detection device side for compensation. In general, there is an aspect that the detection becomes harder as a difference between the detecting voltage and the induction voltage becomes larger. In addition, there is a case where a voltage at the middle point of the superconducting coil becomes higher than the voltage at both ends of the coil. Therefore, in view of avoiding an excess voltage, there generally is a case where a protective circuit that refluxes a coil current for each section obtained by dividing at the middle point is configured, and the resistance and the semiconductor element are included in each circuit to control the maximum voltage.
As an example of detecting the quenching by a signal other than a coil voltage, there is known a method of monitoring a pressure change of a cooling medium in a case of the superconducting coil using a force-cooled conductor. Although this method can resolve many problems in a case of detection by the coil voltage, it is difficult to apply the method to the superconducting coil which is immersion-cooled in the cooling medium.
On the other hand, as a technique to cancel an induction component of the coil voltage, there is known a method of measuring the induction component of the coil voltage by a pick-up coil, and the measured induction component is subtracted from the coil voltage being electrified (refer to Patent Document 3). There also is a case where the induction component of the coil voltage is directly calculated from a current signal without using the pick-up coil. Although these methods are mainly applied to an AC loss measurement of the superconducting coil, the methods may be applied to the quench detection.
It is also known that, in a case where reinforcing metal which is electrically insulated from a coil conductor is wound together with the coil conductor in order to improve rigidity of the superconducting coil, the reinforcing metal can be used as the pick-up coil. In this case, an inductance of the pick-up coil excellently matches with an inductance of the superconducting coil, and then, an ideal pick-up coil is obtained (refer to Patent Document 4).
Patent Document 1: Japanese Patent Application Laid-Open Publication No. Hei 6-333739
Patent Document 2: Japanese Patent Application Laid-Open Publication No. Hei 9-260130
Patent Document 3: Japanese Patent Application Publication No. Hei 5-58246
Patent Document 4: Japanese Patent Application Publication No. Hei 6-56811